1. Field of the Invention
This invention pertains to actuators for opening and closing valves, and in particular to an actuator using a gas turbine motor.
2. Description of the Related Art
Various actuators are used to open and close valves, and for some applications it is convenient to use a valve actuator with a gas turbine motor, which is referred to as a gas motor actuator. Typically, a gas motor actuator uses gas directly from a pipeline for opening and closing a valve in the pipeline. The actuator is adapted for directly connecting to the valve gearhead, and an output shaft on the actuator is connected to an existing hand wheel shaft. The gas motor actuator includes a gas turbine motor and an assembly of gears.
High-pressure pipeline gas is filtered, lubricated and fed to the gas turbine motor for rotating the motor, which develops a rotational power output for opening or closing a valve. The gas turbine motor has two ports, and gas is fed to one port, passes through a housing, rotating the turbine clockwise and discharges through the other port. The rotational output developed is used to move the valve from, for example, an opened position to a closed position. To move the valve from the closed position to an open position, the rotational output is reversed by reversing the flow direction of the gas by feeding it to the second port and discharging it from the first port.
A system of components typically controls when gas is introduced to the gas turbine motor and when gas flow to the motor is stopped. The flow of gas to the gas turbine motor should begin when a signal is received to either open or close the valve. The gas flow should continue until the valve has either moved from an open position to a closed position or from the closed position to the open position. Finally, gas flow should stop when the valve reaches its fully-opened or fully-closed position, which is the limit of the valve travel. If the valve were to travel beyond this limit, then the valve can be damaged.
One gas motor actuator includes the following components. A selector spool valve is coupled to a gas turbine motor, which feeds gas to and discharges gas from the gas turbine motor for opening and closing an actuated valve. The selector spool valve can be operated manually or by a remote signal to position the selector spool valve to one of three positions: open, close or neutral. The "open" position is for opening an actuated valve; the "close" position is for closing the actuated valve; and the "neutral" position is for neither opening nor closing the actuated valve. When the selector valve spool is placed in the "open" position, high-pressure gas can be fed through the selector spool valve into the first port of the gas turbine motor for opening the actuated valve. The gas then rotates the turbine to develop a rotational output from the gas turbine motor, and the gas is discharged from the second port of the gas turbine motor into the selector valve spool for discharge as an exhaust.
To develop an opposite rotational output from the gas turbine motor, the selector valve spool is placed in the "close" position. When the selector valve spool is in the "close" position, high-pressure gas is passed through the selector valve spool into the second port of the gas turbine motor, where it rotates the gas turbine and discharges through the first port of the gas turbine motor into the selector valve spool for discharge as an exhaust. If the rotational output developed by the gas turbine motor was clockwise when the selector valve spool was in the "open" position, then the rotational output that is developed is counter-clockwise when the selector valve spool is placed in the "close" position. The "neutral" position of the selector valve spool blocks gas flow to either port of the gas turbine motor. Thus, by placing the selector valve spool in the "open" or "close" position, the gas turbine motor rotates until gas flow through the selector valve spool is stopped.
Gas flow to the selector valve spool is stopped when a limit is reached. A limit is reached when the valve moves to a fully-opened or a fully-closed position. A gear assembly is used to indicate when a limit is reached. The gear assembly includes a cam gear and a cam shaft, and cams are threadedly engaged with the cam shaft. As the gas turbine motor rotates, the gear system rotates, which rotates the cam shaft. The cams are adjusted to indicate when the valve is fully opened and fully closed. An open limit valve spool is engaged by one of the cams to indicate when the valve is in a fully-opened position. A close limit valve spool is operatively engaged with the other cam to indicate when the valve is in a fully-closed position. The limit valve spools are used to shut off flow of gas to the selector valve spool when the pipeline valve is at a fully-closed or fully-opened limit.
Thus, high-pressure gas, referred to as power gas, can flow through each of the limit valve spools to the selector valve spool. If, for example, the pipeline valve is fully closed and a signal is received by the selector valve spool to move the pipeline valve to an open position, then power gas flows through the open limit valve spool to the selector valve and through the gas turbine motor to open the pipeline valve. The pipeline valve continues to open until a limit is detected by contact between the cam and the open limit valve spool, which closes the open limit valve spool to stop power gas flow through the open limit valve spool and, consequently, through the selector valve spool. Thus, the pipeline valve is moved from the fully-closed position to the fully-opened position and no farther.
Both limit valve spools and the selector valve spool use O-rings as seals. The O-rings slide over ports where power gas is introduced to or discharged from the spool valve. As the O-ring passes over the port, the O-ring expands into the port, and as it slides past the port, it is cut by a wall forming the port. Consequently, operation of these spool valves deteriorates the O-ring seals. When the O-ring seals are deteriorated or destroyed, power gas blows by and is discharged to the atmosphere, which is an undesirable release of gas from the pipeline into the atmosphere. Further, a relatively large amount of power gas is required to open or close a pipeline valve, which is also a release of pipeline gas into the atmosphere.
Another gas motor actuator includes open and close limit spool valves, but rather than a selector spool valve, a pair of poppet valves are coupled to a shuttle valve. One poppet valve is used to open a pipeline valve, and the other poppet valve is used to close the pipeline valve. For remote operation, a pilot gas is used to open the poppet valves. The poppet valves are operatively coupled to a shuttle valve, which is operatively coupled to the gas turbine motor.
When, for example, a pipeline valve is in a fully-closed position and a pilot gas signal is received by the open poppet valve to open the pipeline valve, then power gas passes though the open poppet valve to the shuttle valve and then through the gas turbine motor. The power gas discharges from the gas turbine motor into the shuttle valve and is discharged into the atmosphere. The pipeline valve continues to move from its fully-closed position towards its fully-opened position until a limit is reached. This gas motor actuator requires a relatively large amount of pipeline gas to open or close a pipeline valve. Further, the limit valve spools have the same problem of cutting O-rings as described above.
A different configuration for a gas-operated valve actuator is described in U.S. Pat. No. 4,380,325 issued to Palmer. High-pressure gas or power gas flows through a three-position selector spool valve. The selector spool valve has an "open" position, a "close" position, and a "neutral" position, which are referred to as "forward," "reverse" and "neutral" positions, respectively, for opening, closing and holding the actuated valve in its current position, respectively. When the selector valve is in the open or forward position, power gas flows through the selector spool valve to a forward poppet limit valve. Provided a limit has not been reached, the power gas flows through the poppet limit valve and into a gas turbine motor. The power gas discharges from the gas turbine motor into and through a close or reverse poppet limit valve and into a shuttle exhaust valve.
The forward poppet limit valve is held open against the flow pressure of the power gas by a tripping mechanism. When a limit is reached, indicating the pipeline valve is in a fully-opened position, the tripping mechanism is tripped, allowing the forward poppet valve to close due to the flow pressure of the power gas. This stops the flow of power gas to the gas turbine motor when a limit is reached. To close pipeline valve, the selector spool valve is placed in the close or reverse position, and the gas flows in a direction that is opposite that described above for opening the pipeline valve. The selector spool valve also has O-rings that pass over ports, which causes deterioration of the O-ring seals in the selector spool valve as described above.